The present invention relates to an apparatus for measuring instantaneous values of pressure in the cylinders of a reciprocating engine and/or for computing the mean indicated pressure by integration of the pressure as a function of the piston travel; the apparatus includes transducer means for sensing said pressure at moments determined by sensing means co-operating with markings provided on a part of the drive system of the engine and electronic calculator means to which said transducer and said sensing means are coupled.
A prior art device of this above mentioned type (it is delivered by the Norwegian firm Autronica A/S in Trondhjem) includes a storage oscilloscope for plotting for each cylinder its dynamic characteristic or indicator diagram, which shows the interrelation between the cylinder pressure and the piston travel, and a display means for displaying the mean indicated pressure computed by means of the electronic calculator means and the pressure at predetermined points of said indicator diagram such as the compression pressure and the maximum combustion pressure.
In said prior art device the markings determining the sensing moments of the pressure transducer are provided on a circumferentially disposed band strapped on one end of the crankshaft in such a way that the sensing is taking place at predetermined angular positions of said one end of the crankshaft. Consequently, an electronic device is required, which for each cylinder is able to perform a transformation of the actual crankshaft angle to the corresponding piston position, which measure complicates the device as a whole. The above prior art device has a further important disadvantage in that during operation of the engine the crankshaft is subjected to torsional deformations, part of which are static deformations, the magnitude of which depends on the actual load of the engine and which generally increase with the distance from the band containing the markings to the cylinder, in which the pressure is measued, and part of which are dynamic deformations, which are due to the torsional vibrations of the crankshaft, the amplitudes of said vibrations being dependent on the load as well as on the rate of revolutions. The hereby implied indefiniteness of the relation between the crankshaft angle at the markings and the position of the piston in the individual cylinders leads to a corresponding inexactitude of the measured values read.